(core): `cdk synth` always synthesizes every stack

## :question: General Issue

I have multiple stacks in my project and want I deploy only one of them all other stack assets must be present in file tree or else I get `ENOENT: no such file or directory, stat...`.

I did a little digging and figured:
a. cdk cli spawns process `ts-node /bin/script.ts`
b. cdk cli does not pass stack name to aws-cdk, therefore I cannot know which should be synthesized
c. aws-cdk generates all cloudformation jsons for all stacks it can find and cdk cli picks the ones it needs

I think it would be smart to provide the name of stack and generate only provided one, it would be faster and would not require all assets to be present.

PS.
With aws-cli I'm referening to aws-cdk installed globally and with aws-cdk I refer to project dependency.

### The Question

1. I wonder what was the reasoning for current approach and should it be changed?
2. Maybe I'm missing something and this can be easily resolved?

### Environment

- **CDK CLI Version: 1.27.0 (build a98c0b3)
- **Module Version: "@aws-cdk/core": "^1.27.0"
- **OS: MacOs Mojave
- **Language: Typescript

### Other information

Start by tracing the CDK CLI invocation described in the issue (`ts-node /bin/script.ts`) and how it selects synthesized stacks. The issue proposes passing the requested stack name so only that stack is synthesized; done would mean deploying one stack no longer requires unrelated stacks' assets to be present. The discussion is old, so check for any later decision before starting.

Written by the indexing model from the issue text.